The main difference between a person suited for a 3-4 outside linebacker vs a 4-3 outside linebacker is their ability to get off the ball and play at the line of scrimmage. Starting with a head of steam from 4-5 yards away is very very different from lining up across from a man and beating him off the ball. This takes more initial burst and leverage. While blitzing from the second level takes timing and agility and speed.
At almost all levels your DEs are the core of your defense. Coaching pee-wee through college I can say my first pick would be a left defensive end (left because most QBs are right handed). You don't know how many stud players I have tried to "turn" into a DE but they eventually went back to linebacker. I call these players my "thinkers". At 4-3 outside linebacker you want your "thinkers" there. These are players who read the ball well, analyze plays, reacts to blocking patterns, reacts to route patterns and so on.
Now your 3-4 outside linebacker is a "doer". This guy just wants to go. You can line him up on the line and he will play like a D-lineman. You put him in coverage though and you usually have two options. He can cover the short flat or he can man up on a TE or back. Put him in space he will think too much. You don't want your attack dog thinking when a crime is taking place - and a team trying to get a yard on my defenses is a crime!
There are very very few players I have coached that are both "thinker"s and "doer"s. Well maybe there were a lot but there were very very very few that were really good players that could do both. If I labeled you a "thinker" or a "doer" those are big skills and you were a good player, and if you were a hybrid you were probably 2nd-3rd string.
Some other notes:
some 3-4 outside LBs can move to DE. Most of the time they were picked to OLB position because of their size. If their penetration outweighs their lack of size then they may merit a move up to a 4-3 DE if schemes change. You will hardly ever see a OLB move to a 3-4 DE position which is generally a two gap technique (and "doer"s can't do 2-gap).
some 4-3 DE have moved to OLB. If a DEs size is being taken advantage of they may move to an OLB position in a 3-4. Every once in a while they may transition to a 4-3 OLB but that is basically saying they aren't good at getting after the QB.
a 3-4 OLB is not a glorified DE. He may line up 90% of the plays during a specific game like a DE but that is just that game. What happens in the NFL if an OLB does this (I have never coached a 3-4 and have very very rarely seen it used in college) and what I would do, is simply pass every down. You keep your 3 big bodies with 2-gap technique with no exotic blitzes (because the left OLB is just their left DE) and I will throw laddered slants to your left side all day. If you do this you are leaving potentially your corner, safety, and ILB on my TE, RB, and WR. This doesn't compute right? If your corner doesn't play shallow zone my TE and RB catches outs all day. If your corner does play a shallow zone my WR runs streaks all day (and when your safety starts cheating posts).
a 3-4 OLB is a hybrid that can play on the line or at the second level. With a good schemed 3-4 an OLB may "blitz" (play as a DE) maybe 30% of the time. Now if they have a very favorable matchup maybe 80-90%. How do I get to 30%? Well you have 2 OLBs. That would cover 60% of the snaps. You will rush 3 on maybe 15-20% of the snaps. The others should be filled with ILB or safety blitzes.
this question is really around pro football. The 3-4 isn't really relative at any other level. Some FCS schools run the 3-4 sometimes. To run it right not only do you have to have 3 stud DL to run 2-gap but you also have to have intelligent players that understand all the scheming involved to run a 3-4 right. It isn't that players can't or aren't smart enough it is just that only at the pro level is there enough time involved to learn how to do it without opening up glaring holes because of assignment mistakes.
at most an OLB in a 4-3 will blitz 10-15% of the time. Generally your 4-3 OLB has to be faster because they need to cover a 3rd of the field instead of a 4th - this is a HUGE difference. Also generally speaking size isn't much of a concern for a 4-3 OLB with some major colleges playing guys around 200 pounds (and lighter).
if you have a 4-3 OLB that gets after the QB well then chances are most of their pass rushes have them coming in as basically a DE in nickel and dime packages. Note that this is more of a high school or college thing. If an OLB rushes the passer well in the pros he moves to a 3-4 team or to DE so he makes more money.
Special Note: For those with ****ass high school coaches that ran a 3-4. Chances are you had one of your OLB line up on the line (who acts like a DE), probably on the weak side. Then you have the other 3 LBs plus 2 corners on the second level. Then you had a free safety, then a "rover" or some other sort of roaming strong safety.
Guess what? That isn't a 3-4. That is a 4-3 in Cover 1 (or simply a 4-4 with the SS assuming role of OLB). High school coaches like to say they are running a 3-4 to make themselves feel current or smart. In 25+ years of coaching I have not seen a true 3-4 run at the high school level. Even with the fake 3-4 it puts undo pressure on your corners and even if you have an athlete advantage over me I can probably scheme my guys open (yes we would pass most downs).